Quaternary Movements on an Inherited Mesozoic Structure: The Rush Peak Fault, West-Central Idaho

Publication Date


Type of Culminating Activity


Degree Title

Masters of Science in Geology



Major Advisor

Spencer H. Wood


The Cuddy Mountains region of west-central Idaho has undergone multiple episodes of deformation from Mesozoic crustal assembly by terrane accretion to late Quaternary faulting. This study focuses on the Cenozoic faulting and its interactions with the Mesozoic crustal architecture along the southern Cuddy Mountains. The Rush Peak fault is an east-northeast striking, 12-km long fault that juxtaposes Mesozoic granitic basement on the north against Miocene volcanic rocks of the Columbia River Basalt Group on the south vertical displacement is greater than 500 m in the vicinity of Rush Creek. A shear zone observed in Rush Creek canyon within the Mesozoic plutonic basement of the Olds Ferry terrane trends parallel to the nearby Rush Creek fault. Anastomosing cm-dm thick mylonitic shear bands collectively comprise this shear zone which is approximately 50 m thick. Individual mylonitic bands generally strike N50̊ to 80̊E and incline steeply to moderately to the southeast. Outcrop observations of shear sense indicators and petrographic evidence (S-C fabric and C' shears) suggest sinistral strike-slip movement. Slickensides and slickenfibers on subvertical east-northeast striking fractures are commonly seen in Mesozoic rocks adjacent to the Rush Peak fault trace. Kinematic analyses of these features also indicate sinistral strike-slip movement in association with fracture development. Further evidence of reactivation along the southern Cuddy Mountains is the emplacement of a rhyolitic dike approximately 30-50 m wide that parallels the Rush Peak fault for the fault's entire length. However, there is no evidence of deformation of the dike, therefore, it is interpreted to be Tertiary(?) in age.

The Rush Peak fault is distinctive in that it trends east-northeast following the trend of the shear zone in a dominant north-northwest Cenozoic tectonic regime. The trend of the Rush Peak fault suggests that the current tectonic regime is taking advantage of the relatively weak nature of the older shear zone and has reactivated the southern Cuddy Mountains in a brittle mode. In Rush Creek canyon, two parallel scarps separated by about 20 m displace Quaternary alluvium and are coincident with the trace of the Rush Peak fault. One scarp near Rush Creek cuts late Quaternary alluvium vertically approximately 3.5 m and left-laterally displaces a boulder levee approximately 6 meters. Weathering rinds of Columbia River Basalt cobbles in the "B" horizon in the boulder levee have thicknesses of 0.047 to 0.77 mm, consistent with a depositional age of around 16 ka. The Rush Peak fault, therefore, has been active with oblique left-lateral slip in the late the Quaternary. Quaternary fault scarps have not yet been documented elsewhere along the fault, although the fault marks a sharp break in slope for a distance of 12 km. The average slope of the southern face of Cuddy Mountain is 21 degrees, whereas the average slope of the basalt surface across the fault to the south is 11 degrees. Stream-gradient values were calculated along the southern range-front to find zones of late Quaternary movement. Stream-gradient index (SL) values were categorized by stream order and separated into low, medium-low, medium-high, and high zones. SL zones align well with the mountain front and fault trace. Knickpoints on third-order streams have migrated approximately 0.5 km up gradient from the fault zone.

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